1. Field of the Invention
This invention relates generally to semiconductor fabrication technology, and, more particularly, to a semiconductor device formed over a multiple thickness buried oxide layer, and methods of making same.
2. Description of the Related Art
There is a constant drive within the semiconductor industry to increase the operating speed of integrated circuit devices, e.g., microprocessors, memory devices, and the like. This drive is fueled by consumer demands for computers and electronic devices that operate at increasingly greater speeds. This demand for increased speed has resulted in a continual reduction in the size of semiconductor devices, e.g., transistors. That is, many components of a typical field effect transistor (FET), e.g., channel length, junction depths, gate insulation thickness, and the like, are reduced. For example, all other things being equal, the smaller the channel length of the transistor, the faster the transistor will operate. Thus, there is a constant drive to reduce the size, or scale, of the components of a typical transistor to increase the overall speed of the transistor, as well as integrated circuit devices incorporating such transistors.
As transistors are continually scaled in keeping with the requirements of advancing technology, device reliability dictates a concomitant reduction in the power supply voltage. Hence, every successive technology generation is often accompanied by a reduction in the operating voltage of the transistor. It is known that transistor devices fabricated on silicon-on-insulator (SOI) substrates exhibit better performance at low operating voltages than do transistors of similar dimensions fabricated in bulk silicon substrates. The superior performance of SOI devices at low operating voltage is related to the relatively lower junction capacitances obtained on an SOI device compared to a bulk silicon device of similar dimensions. The buried oxide layer in an SOI device separates active transistor regions from the bulk silicon substrate, thus reducing junction capacitance.
Transistors fabricated in SOI substrates offer several performance advantages over transistors fabricated in bulk silicon substrates. For example, complementary-metal-oxide-semiconductor (CMOS) devices fabricated in SOI substrates are not prone to disabling capacitive coupling, known as latch-up. In addition, transistors fabricated in SOI substrates, in general, have large drive currents and high transconductance values. Also, the sub-micron SOI transistors have improved immunity to short-channel effects when compared with bulk transistors fabricated to similar dimensions.
Although SOI devices offer performance advantages over bulk silicon devices of similar dimensions, SOI devices share certain performance problems common to all thin-film transistors. For example, the active elements of an SOI transistor are fabricated in a thin-film active layer. Scaling of thin-film transistors to smaller dimensions requires that the thickness of the active layer be reduced. However, as the thickness of the active layer is reduced, the electrical resistance of the active layer correspondingly increases. This can have a negative impact on transistor performance because the fabrication of transistor elements in a conductive body having a high electrical resistance reduces the drive current of the transistor. Moreover, as the thickness of the active layer of an SOI device continues to decrease, variations in the threshold voltage (VT) of the device occur. In short, as the thickness of the active layer decreases, the threshold voltage of the device becomes unstable. As a result, use of such unstable devices in modern integrated circuit devices, e.g., microprocessors, memory devices, logic devices, etc., becomes very difficult if not impossible.
The present invention is directed to a device and various methods that may solve, or at least reduce, some or all of the aforementioned problems.
The present invention is generally directed to a semiconductor device formed over a multiple thickness buried oxide layer, and various methods of making same. In one illustrative embodiment, the device comprises a bulk substrate, a multiple thickness buried oxide layer formed above the bulk substrate, and an active layer formed above the multiple thickness buried oxide layer, the semiconductor device being formed in the active layer above the multiple thickness buried oxide layer. In a more particular embodiment, the multiple thickness buried oxide layer further comprises a first section positioned between two second sections, the first section having a thickness that is less than the thickness of the second sections.
In one illustrative embodiment, the method comprises performing a first oxygen ion implant process on a silicon substrate, forming a masking layer above the substrate after the first oxygen implant process, performing a second oxygen ion implant process on the substrate through the masking layer, and performing at least one heating process on the substrate to form a multiple thickness buried oxide layer in the substrate. In another illustrative embodiment, the method comprises forming a masking layer above the substrate, performing a first oxygen ion implant process on a silicon substrate through the masking layer, removing the masking layer, performing a second oxygen ion implant process on the substrate after the masking layer is removed, and performing at least one heating process on the substrate to form a multiple thickness buried oxide layer in the substrate.
In yet another illustrative embodiment, the method comprises forming a layer of silicon dioxide above a first substrate, forming a masking layer above a portion of the layer of silicon dioxide, performing at least one etching process to etch a recess in the substrate adjacent each side of the masking layer, and removing the masking layer. The method further comprises performing at least one of an oxidation process and a deposition process to form silicon dioxide in at least the recesses, performing at least one chemical mechanical polishing operation on at least the silicon dioxide formed in the recesses, bonding a second substrate to at least the silicon dioxide formed in the recesses, and removing a portion of the second substrate.